CRAYFISH BURROWS FROM THE LATEST CRETACEOUS LOWER CANTWELL FORMATION (DENALI NATIONAL PARK, ALASKA): THEIR MORPHOLOGY AND PALEOCLIMATIC SIGNIFICANCE

The latest Cretaceous Lower Cantwell Formation of the central Alaska Range contains an abundance of megafloral remains and invertebrate and vertebrate trace fossils. Though dominated by herbivorous dinosaur footprints, the abundance and diversity of fossil bird tracks are unique. Crayfish burrows, can now also be included as they occur from several areas along a 50 km transect in Denali National Park. Most crayfish burrows from the Cantwell Formation are preserved only in cross-section and range from approximately 5–10 cm in diameter. Where preserved in full relief, burrow depth is generally < 50 cm. Burrow morphology is similar to burrow morphology of modern freshwater crayfish (Cambaridae).

The Cantwell Formation fills the Cantwell Basin, a 135 km-long and up to 35 km-wide, east–west trending basin, bracketed by the Hines Creek Fault to the north and the McKinley Fault to the south. Basin fill comprises up to 4000 m of continental deposits, predominantly including braided rivers, alluvial fans, floodplains, and ponds.

Crayfish burrows from the Lower Cantwell Formation, a Cretaceous high-latitude paleoecosystem, provide evidence of moisture fluctuations, as well as insight into the mean annual temperature experienced at the time. The Cretaceous mean annual temperature was more like that of southern-most Ontario, Canada, where the northern-most burrowing crayfish are found today. The burrow depth suggests (1) no permafrost was present and (2) the phreatic zone was ~30–50 cm below the paleo-ground surface. The paleoclimate is interpreted as humid continental (Köppen scheme) with average summer high temperatures between 25^oC to 28^oC and average low temperatures between -6^oC to 0^oC, estimates that compare somewhat favorably with previous CLAMP estimates of a warm monthly mean of 17.08 +/- 1.6 ^oC and a cold monthly mean of -2.31 +/- 1.9 ^oC.